The present invention relates to a glass panel for a color television picture tube used for a television receiver and the like, and particularly relates to an improved technology for seal portions, formed on an inner wall surface of a skirt portion of the panel, for sealing stud pins for retaining a shadow mask.
As is generally known, major glass parts used for the color picture tube comprise a panel on which an image is projected, a funnel, and a neck in a tube shape with a small diameter. As shown in FIG. 7, a panel 1 comprises a substantially rectangular face portion 2 with an effective screen for showing an image, and a skirt portion 4 continuous to a periphery of the face portion 2 through a blend R portion 3. At an open end of the skirt portion 4, a seal end surface 5 is formed for being joined to a funnel.
As shown in FIG. 8, in a color television picture tube manufactured using the panel 1, the panel 1 and a funnel 6 are sealed together through flit glass 7 interposed between the seal end surface 5 of the panel 1 and a seal end surface 6a of the funnel 6, and a fluorescent film 8 and a metal back coating 9 are formed on an inner surface of the face portion 2 of the panel 1 while laminating them in this order. A shadow mask 10 is provided inside the panel 1, and multiple metal stud pins 12 are sealed and embedded in an inner wall surface 11 of the skirt portion 4 of the panel 1. The stud pins 12 and support springs 13 engaged with the stud pins 12 support the shadow mask 10 inside the color television picture tube.
In this type of a shadow mask color television picture tube, an electron gun provided inside the neck irradiates an electron beam, and the electron beam passes through holes 14 formed at a portion of the shadow mask 10 opposed to the face portion 2 after being directed by a deflecting yoke. The passing electron beam is properly irradiated on the fluorescent film 8 of three colors comprising R, G, and B, and as a result, an image is displayed. Thus, it is necessary that the fluorescent film 8 and the shadow mask 10 are disposed and opposed to each other at predetermined positions at a predetermined high precision for providing a proper image receiving action.
To meet this requirement, because it is necessary to seal the stud pins 12, which are jigs for retaining the shadow mask 10, vertically with respect to the axis of the picture tube at a high precision, and simultaneously to maintain the stud pins 12 and openings of the support springs 13 in a constant engaging state, extremely high reproducibility is necessary for a dimensional accuracy when the stud pins 12 are sealed on the inner wall surface 11 of the skirt portion 4 of the panel 1.
In this case, a support tool and pin seal means are used to seal the stud pins on the panel (not shown). The pin seal means is provided with a ceramic holder portion and ground coil portion. The ceramic holder portion approaches to and departs from the inner wall surface of the skirt portion of the panel while holding the stud pin to insert the stud pin. The ground coil portion is used to heat the stud pin held by the holder portion with high frequency energy. As a specific procedure for the sealing, a mold is used to press-form a panel into a desired shape in a molding process for the panel glass, and the support tool is used to hold the panel on a predetermined table immediately after the panel is cooled down and solidified to about 400° C. to 450° C. Then, the ceramic holder portion inserts the stud pin heated up to 1100° C. to 1250° C. by the ground coil portion of the pin seal means into the inner wall surface of the skirt portion of the panel. After this, the panel transferred form the pin seal means is transported to a lehr, to anneal the pin-seal portions along with the panel.
Because the inside of the color television picture tube is in a high vacuum state, a pressure resistance for resisting the vacuum is necessary for the panel 1. However, because this type of the color television picture tube has a complicated shape formed by joining a funnel to a panel having a rectangular face portion, its vacuum stress distribution is consequently complicated.
Specifically, because the highest vacuum tensile stress is generated on a connection portion between the peripheral of the face portion 2 to the skirt portion 4 through the blend R portion 3 of the panel 1, a neighborhood of the blend R portion 3 of the skirt portion 4 is formed thick for withstanding the high stress. On the other hand, because the vacuum tensile stress relatively lower than that on the panel 1 is applied on the funnel 6 so that the funnel 6 is relatively formed thin, the thickness of the seal end surface 5 of the panel 1, which is joined to the seal end surface 6a of the funnel 6 without generating a step, is thinner than the thickness of the neighborhood of the blend R portion 3 of the skirt portion 4.
This causes such a thickness distribution at the skirt portion of the panel 1 that the thickness gradually decreases from the neighborhood of blend R portion 3 to the seal end surface 5, and consequently, the inner wall surface 11 of the skirt portion 4 presents an inclined surface which departs from a center axis of the picture tube (a panel center axis) mare at the seal end surface 5 than at the face portion 2.
When the pin seal means is used to seal the stud pin 12 in the inner wall surface 11 of the skirt portion 4, the inclined inner wall surface 11 of the skirt portion 4 causes the following problems. Namely, because the inner wall surface 11 is inclined, it is extremely difficult to evenly form a glass buildup all around the stud pin 12 for forming a pin-seal portion with high strength while the stud pin 12 with the diameter of 10 mm to 15 mm is maintained in an attitude vertical to the panel center axis. This causes a generation of a crack due to a defective seal such as an inclination of the stud pin 12 after sealing, and consequently a breakage of the glass panel, and because the stud pins 12 are not accurately sealed vertically with respect to the panel center axis, it is also difficult to retain the shadow mask highly accurately.
To meet these problems, flat surfaces 15 are formed on the inner wall surface 11 as a conventional measure. The flat surfaces 15 are substantially parallel with a first direction (a direction a—a in FIG. 7) along the panel center axis X, and substantially parallel with a second direction (a direction b—b in FIG. 7) along a line across two corners 4R neighboring to each other of the skirt portion 4. However, there is no consideration given to not only an outer peripheral shape of a pin-seal portion 16 having the flat surface 15 seen from the opposing inner wall surface 11, but also to a recess and protrusion shape of the pin-seal portion 16 seen from the seal end surface. As a result, conventionally, the outer peripheral shape of the flat surface 15 is merely formed as substantially rectangular as shown in FIG. 9(a), and inclined portions 16a rising sharply are formed at an outer periphery in the second direction of the flat surface 15 as shown in FIG. 9(b) so that the flat surface 15 merely protrudes toward the opposing inner wall surface 11.
As a result, corners of the flat surface 15, especially corners 16x on the side of the face portion 2 are not obtuse angle corners, but substantially right angle corners, and the sharply rising inclined portions 16a between the outer periphery in the second direction of the corners 16x and the inner wall surface 11, in other words, steps gradually rising toward the seal end surface 5, exist.
In this case, when the glass panel having flat surfaces 15 in the shape described above is molded, mold surface portions in substantially rectangular recess shapes corresponding to shapes of the pin-seal portions 16 are formed on a plunger (a male mold), a molten glass mass called as glass gob is supplied for a bottom mold to which a shell mold is attached, in this state, the plunger is pressed against the gob to apply a pressure, thereby the molten glass flows up from the face portion to the skirt portion through the blend R portion.
When the corners 16x of the flat surface 15 on the side of the face portion 2 are substantially right angle, and there are the steps between the corners 16x and the inner wall surface 11, because the corners 16x inhibit a smooth flow of the molten glass when the molten glass rises up to the skirt portion, and there exist the large local thickness changes, the temperature of the plunger largely differs between at portions corresponding to the flat surfaces and at a portion corresponding to the inner wall surface. This generates defective molding such as wrinkles, and small recesses and protrusions around the flat surfaces of the molded glass panel, especially around the corners 16x on the side of the face portion 2, and this not only decreases the commodity value, but also causes the decrease of the yield.